What To Do Next?
Professor Powrie’s particular expertise lies in the study of railway embankments and cuttings. He says the landslips at Harbury (on the Chiltern route) and more recently at Wrecclesham, are “probably” evidence of the impact of a changing climate. It’s not possible to blame individual specific events on the weather patterns, he warns. But the increasing frequency of landslips suggests a relationship.
“Embankments and cuttings are having to handle patterns of weather which are much wetter than in the past. To do anything about it, such as improving the drainage, is incredibly expensive.
“Embankments on clay are key. If you leave trees on them you get a cycle of shrinkage and swelling. If you take the trees away you get a gradual rise in the pore water pressure. If the water pressure is too high you get a potentially major slip… as we have seen in recent winters.
“The downside of trees is that they blow down in front of trains. They cause leaves on the line. But the upside is that the roots of the trees are effective in taking moisture out of slopes, and that will help with stability.
“In a normal weather pattern we can do enough to make sure that a bank remains stable, leaving trees over the top or bottom third of a slope. The question really is whether you try to make the embankments resilient enough to stand a 1-in-50-year event, or a 1-in-100-year event.”
NR’s Mike Gallop responds: “Every day I fund vegetation clearance and drainage improvement. We manage in a sustainable way for condition and performance, rather than for ecology. I have 15,000 earthworks on the Western route. Each has an inspection regime relevant to its risk. This is the asset register of our embankments.”
But does his vegetation management take into account future climate change?
“No. We don’t at the moment. But where embankments are renewed or where we have a slip, we design to modern standards. Within that is a higher safety factor reflecting an element of future-proofing for rainfall and flooding. We are becoming wetter, so we are increasing the size of culverts and drainage ditches.”
Powrie sees damage to the Lamington viaduct on the West Coast Main Line as further evidence of climate change. It took Network Rail seven weeks to repair the bridge over the River Clyde, where scouring by flood water came close to a catastrophic destruction of the foundations last Christmas. It was described as “a race against time”, with the river flow diverted so that engineers could install new steel bearings and a concrete plinth.
Says Powrie: “Add it all up: scour from rivers, effects on slope stability, vulnerable coastal sections, expansion and contraction of the rail leading to buckling in hot weather - there are lots of significant impacts and you can see them right now. Most years the railway will be OK, but in some years it will be caught out.”
A 2014 review of transport resilience by the Department for Transport noted that most railway earthworks were hand-built by the Victorians, and poorly engineered by today’s standards. It identified the 11,300 miles (18,200km) of embankments and cuttings as a particular risk. Some 105 earthworks failed in the winter of 2013-2014, some of them resulting in lengthy line closures.
The review concluded: “It would be both very difficult and prohibitively expensive to ensure total physical resilience, so it is equally about ensuring processes and procedures to restore services and routes to normal as quickly as possible after extreme weather events have abated.”
The 2015 Harbury landslip closed a route used by 50 freight trains and 80 passenger trains each day.
“I’m not sure whether we have given up on mitigation,” says Powrie. “In terms of trying to stop it from happening, we must continue to encourage people to travel by rail rather than by road, because that is better for the climate.
“But we have to accept that a considerable degree of climate change is inevitable, and therefore we must be talking more about adaptation. People will have to be more tolerant of the consequences when there is an extreme event that disrupts the railway.
“You can only go so far to make the railway climate change-proof. You have to accept that in an increasing number of circumstances there will be restricted speed running and sometimes a suspended service. That is just an economic fact of life.”
Powrie explains: “Quite often now the railway is protecting other infrastructure. In the meadows south of Oxford, the railway is really acting as a barrier between flooded fields on one side and a housing estate on the other. Dual-purpose infrastructure which is both more resilient and which is potentially a flood defence or a means of water storage - that is something we can work on.
“Another example is the overhead line equipment. A problem with the Great Western modernisation has been specifying modern, chunky masts that attract more wind, ice and snow. That has required increased foundation size to take the weight.
“Much can be done around reducing the environmental loadings on structures to give economy. The trick is to make those structures more climate-proof. That does not mean making them bigger - bigger attracts greater loads. We have to make them cleverer - more aerodynamic and therefore unable to accumulate ice and snow. We need to be thinking about that now, and it has not been thought about on the Great Western electrification project.”
Fourteen leading academic institutions have joined the Government-funded UK Collaboratorium for Research in Infrastructure and Cities. As part of that, the University of Southampton is building a £40m National Infrastructure Laboratory that will develop, test and prove new designs. This could include bridge piers that resist scour or greater water velocity, or novel forms of track or structures that are more resilient to changes in climate.
